GPU Rendering on an Intel Iris Pro (Apple Macbook Pro)
As of 2.78 GPU rendering is only supported under certain NVidia graphics cards that support the correct CUDA version.

Starting with 2.79, GPU rendering will introduce support for AMD graphics cards through OpenCL on feature par with CUDA, since basic support was available since 2.75 (thanks to a bunch of patches contributed by AMD itself).

Under Intel it is considered an experimental feature, and as far as I know, is too slow and offers no benefit over regular CPU rendering. You can enable it by setting a global variable with command line but don't expect it to preform acceptably, not even to work reliably at any point.